In recent years, the elevation of the thermal efficiency of thermal power generation has been examined as one of energy saving measures. In order to improve the power generation efficiency of a generator gas turbine, the elevation of the gas inlet temperature is effective, and in some cases, the temperature is elevated to about 1500° C. In order to realize the temperature elevation in power generators in this way, a stationary blade, a moving blade, the wall material of a combustor, or the like which constitute the gas turbine are required to be constituted with heat resistant members. However, although the material of the turbine blades is a refractory metal, it cannot endure such a high temperature. Therefore, a thermal barrier coating (TBC), a laminate of ceramic layers composed of oxide ceramics, is formed on the refractory-metal substrate through a metal bonding layer by a film-forming method such as thermal spraying, so as to protect from high temperatures. As the ceramic layer, a ZrO2-based material, especially a YSZ (yttria-stabilized zirconia) which is ZrO2 partially or completely stabilized by Y2O3, is often used because it has a relatively low thermal conductivity and a relatively high coefficient of thermal expansion, among ceramic materials.
However, it is considered that, if the moving blade, the stationary blade, and the like of a gas turbine are coated with the abovementioned thermal barrier coating comprising the ceramic layers composed of YSZ, the inlet temperature of the turbine can be elevated to a temperature higher than 1500° C. depending on the type of the gas turbine. When the gas turbine is operated at such a high temperature, there has been a concern in which a part of the ceramic layer is spalled away and its heat resistance is impaired during the operation of the gas turbine under severe operating conditions. Moreover, in recent years, it is considered in view of higher efficiency that the inlet temperature of the turbine reaches as high as 1700° C. so that the surface temperature of turbine blades elevates to as high as 1300° C. Therefore, the situation is such that further higher heat resistance has been required for the thermal barrier coating of turbine blades.
The above-described problem of the spalling of the ceramic layer composed of YSZ is caused by that the stability of YSZ crystals is insufficient in a high-temperature environment, and the YSZ crystals do not have sufficient durability against a large thermal stress. That is to say, when thermal cycles involved in the start and shutdown of the turbine are applied, a ceramic layer having a lower coefficient of thermal expansion as compared to a heat resistant substrate and a bond coat layer may be spalled away because of a stress or the like due to the difference in the coefficient of thermal expansion from the heat resistant substrate and the bond coat layer (hereunder, the durability against such an action due to thermal cycles is referred to as “thermal cycle durability”). In order to solve this problem, a thermal barrier coating member using a zirconia layer stabilized by Yb2O3, or a zirconia layer ZrO2 stabilized by Yb2O3 and Er2O3 has been proposed in Patent Document 1.
Patent Document 1:
Japanese Unexamined Patent Application, Publication No. 2003-160852.